System for regulating fluid pump pressures

ABSTRACT

A method is provided for regulating fluid pump pressures by detecting an elevation differential between a fluid flow control device and the distal end of a fluid line in communication with the fluid flow control device. A fluid flow control device, for instance a peritoneal dialysis device, is at a first height, a distal end of a fluid line is at a second height, and a valved outlet, when open, affords communication between the fluid flow control device and the distal end of the fluid line. The elevation differential is correlatable with a pressure measurable during a calibration procedure provided as a part of the methodology.

TECHNICAL FIELD

[0001] The present invention relates to fluid flow control devices and,more specifically, to regulating pump pressures. In particular, theinvention provides a method and apparatus for increasing the fluid flowrate in a fluid flow control device while maintaining desired pressurelevels. The present invention also relates to systems that can determinethe relative elevation of a pump with respect to a distal end of a linein communication with the pump.

BACKGROUND ART

[0002] A function of fluid flow control systems is to regulate the rateof distribution of transport fluid through a line. Some examples offluid control devices are peritoneal dialysis machines and intravenousfluid delivery systems. Fluid flow control systems may include apermanent housing which does not come into direct contact with thetransporting fluid and into which a fluid-exposed disposable cassette isplaced. Flexible membranes, or other structures that respond topressure, maintain separation between the permanent and disposablecomponents. Examples of such control systems and their sub-components(in particular, valves) are disclosed in U.S. Pat. Nos. 4,778,451,4,976,162, 5,088,515, and 5,178,182. These patents are all issued toKamen and are all hereby incorporated herein by reference.

[0003] One problem with respect to fluid flow control devices arises in,for example dialysis treatment. Patients want to minimize the time spenthooked up to the peritoneal dialysis machine. In order to satisfypatient demands, the flow rate of the fluid pumped into the patient'scatheter may be proportionally increased by increasing the pumpingpressure. However, international specifications (for example, EN 50072)regulate the maximum and minimum pressures allowed in the patient'scatheter. The maximum positive pressure allowable is set at 150 mm Hg(˜3 psi), and the minimum (or maximum negative, or suction pressure) isset at −75 mm Hg (˜−1.5 psi). Prior art dialysis machines use pumpingpressures of about 75 mm Hg (1.5 psi) when pumping fluid into thepatient. If the dialysis machine and the patient are at the sameelevation, the pressure applied at the pump will be very close to thepressure at the patient's catheter. If, on the other hand, the dialysismachine is elevated above the patient, the pressure at the patient'scatheter will be higher than the pressure applied at the pump.Consequently, to insure a margin of safety, the pumping pressure is setwell below the maximum allowable pressure to compensate for anyuncertainty in the position of the patient relative to the dialysismachine.

SUMMARY OF THE INVENTION

[0004] A method is provided for regulating fluid pump pressures based onthe relative elevation between a fluid flow control device and a distalend of a fluid line by providing at least one liquid volume in valvedcommunication with the distal end. The pressure measurement of theliquid volume is calibrated, and then valving is opened to establishcommunication between the liquid volume and the distal end of the fluidline. A pressure associated with the liquid volume is measured, and thefluid pump pressure is adjusted in accordance with the measuredpressure.

[0005] Preferably, the fluid flow control device has two liquid volumes.A first liquid volume is in valved communication with a second liquidvolume. The fluid line is preferably in valved communication with bothliquid volumes. The pressures in the liquid volumes are calibrated, andcommunication between one liquid volume and the distal end of the fluidline is established. A pressure associated with the one liquid volume ismeasured, and the fluid pump pressure is adjusted in accordance with themeasured pressure.

[0006] The fluid flow control device preferably includes a controlvolume for each liquid volume, a transducer for each control volume, anda processor for reading and storing pressure values, computing andidentifying a correlation between pressure values, and calculatingpressure values based on identified correlations. The processor mayestimate the elevation differential based upon the pressure values,and/or regulate fluid pump pressures. The fluid flow control device mayalso include pressure means for pressurizing a liquid volume. The devicemay further include one of a wide variety of valve arrangements forcontrolling fluid communication between the liquid volumes and thedistal end of the line. The processor may also control the valvearrangement, the means for pressurizing the liquid volume, and the fluidpump pressure.

[0007] In another preferred embodiment, the liquid volume and thecontrol volume themselves are parts of a pump. Preferably, the pumpincludes a flexible membrane that divides the liquid volume and thecontrol volume. In other embodiments, the fluid flow control deviceincludes a pump.

[0008] In a preferred method for detecting the relative elevationbetween a first location and a second location, a fluid flow controldevice is provided at the first location with at least one membrane pumpin valved communication with the second location. The membrane pump isisolated from the second location, and a pressure transducer of themembrane pump is calibrated. Valving is then opened to establishcommunication between the membrane pump and the second location. Thepressure of the membrane pump is measured, and the relative elevationbetween the first location and the second location is estimated.

[0009] In a further embodiment, calibrating the pressure transducer mayinclude filling the membrane pump with fluid in pressure equilibriumwith the pressure at the first location, measuring a first calibrationpressure of the membrane pump, filling the membrane pump with fluid inpressure equilibrium with a known (i.e., predetermined or measured)calibration pressure, and measuring a second calibration pressure of themembrane pump. The relative elevation between the first location and thesecond location may be estimated based on the known calibrationpressure, the first calibration pressure, and the second calibrationpressure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be more readily understood by reference to thefollowing description, taken with the accompanying drawings, in which:

[0011]FIG. 1 is a perspective view of a prior art fluid flow controldevice;

[0012]FIG. 2 illustrates a membrane-based fluid flow control systemcontained in the device of FIG. 1;

[0013] FIGS. 3(a) and 3(b) schematically illustrate the relationshipbetween the relative elevation of the fluid flow control device and thepressure experienced at the distal end of a fluid line;

[0014]FIG. 4 is a block diagram illustrating the process of detectingthe relative elevation and regulating fluid pump pressure according toone embodiment of the present invention;

[0015] FIGS. 5(a) and 5(b) are block diagrams illustrating calibrationand regulation processes according to another embodiment of theinvention; and

[0016]FIG. 6 is a graphical representation of the relationship derivedfrom the process of FIG. 5.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0017]FIG. 1 shows a prior art fluid flow control device 200. Adisposable cassette 201 is securely mounted onto the fluid flow controldevice 200. Fluid communication with the cassette 201 is maintained froma heated solution bag 202 via a solution inlet line 203 and is alsomaintained to a distal end 208 of an outlet line 204. The fluid flowcontrol device 200 has an occluder bar 205 that when activated by thefluid flow control device 200, occludes both the inlet line 203 and theoutlet line 204. The fluid flow control device 200 is shown to have twopumps 300 and 310, each pump having inlet and outlet valves 306, 307,316, and 317.

[0018]FIG. 2 illustrates a membrane-based fluid control flow systemutilized in fluid flow control device 200 of FIG. 1 and having a firstpump 300 and a second pump 310. Flexible membrane 303 is shown asdividing first pump 300 into a first control volume 301 and a firstliquid volume 302. The first control volume 301 may be pressurizedthrough a first pressure line 304. The pressure in the first controlvolume 301 is measured by a first pressure transducer 305 attached toand in fluid communication with the first control volume 301. Similarly,flexible membrane 313 divides second pump 310 into a second controlvolume 311 and a second liquid volume 312. The second control volume 311may be pressurized through a second pressure line 314. The pressure inthe second control volume 311 is measured by a second pressuretransducer 315 attached and in fluid communication with the secondcontrol volume 311. Pressurizing may occur through the use of a controlgas or liquid, or other methods known in the art, such as pumps,pistons, pressurized reservoirs, valves, and vents. As noted above thesepressurizing devices are explained in greater detail in the U.S. Patentsissued to Kamen and incorporated herein by reference.

[0019] A first outlet valve 306 controls the outlet flow from the firstliquid volume 302 to the outlet line 204 and a second outlet valve 316controls the outlet flow from the second liquid volume 312 to the outletline 204. The outlet flows from the first pump 300 and second pump 310are, therefore, in fluid communication with each other and with thedistal end 208 (see FIG. 1) of the outlet line 204. A first inlet valve307 controls the inlet flow into the first liquid volume 302, and asecond inlet valve 317 controls the inlet flow into the second liquidvolume 312. The inlet flows from the first pump 300 and the second pump310 are in fluid communication with each other and with the heatedsolution bag 202 through solution inlet line 203. When activated, theoccluder bar 205 isolates both the outlet line 204 and the solution bag202 from the pumps 300 and 310 while allowing fluid communicationbetween the two pumps 300 and 310 when either or both sets of valves areopen.

[0020]FIG. 3(a) schematically illustrates a fluid line 204 incommunication with a subject 31 at the same elevation as a fluid flowcontrol device 200. When the distal end 208 of the line 204 and thefluid flow control device 200 are at the same elevation, the pressure atthe distal end 208 of the line 204 is equal to the pressure in the fluidflow control device 200. FIG. 3(b) schematically illustrates a subject31 at a lower elevation to that of fluid flow control device 200. Whenthis situation occurs, the pressure in the distal end 208 of the line204 is greater than the pressure in the fluid flow control device 200.If, for example, the fluid flow control device 200 is a peritonealdialysis machine, the relationship between the relative elevation andthe pressure differential may be calculated. A 0.3 m (one foot)elevation difference between the patient and the dialysis machineresults in about 25 mm Hg (0.5 psi) difference between the pressure inthe dialysis machine and the pressure in the catheter attached to thepatient. Clearly, the lower the patient is in relation to the dialysismachine, the greater the pressure will be in the catheter. Therefore,for the example discussed above, if it were possible to determine therelative elevation of the patient with respect to the dialysis machine,the pumping pressure could be decreased to maintain a margin of safety.Conversely, for the above example, the fluid could be safely withdrawnfrom the patient at a lower pressure (higher negative pressure) andstill maintain the same margin of safety.

[0021]FIG. 4 is a block diagram illustrating the process of detectingthe relative elevation and regulating fluid pump pressure according toone embodiment of the present invention. In this embodiment, the fluidflow control system of FIG. 2 is employed. The pressure of at least oneof the pumps 300 and 310 are correlated with that of the pressure, atthe distal end 208 of the fluid line 204, which, in turn may be relatedto the elevation differential. The correlation is complicated due to thefact that, in membrane-based systems, the flexible membranes 303 and 313store some of the pV work as elastic energy resulting in slightlydifferent pressures across the membrane at equilibrium. Therefore, theinvention provides for measurements, correlations, and the developmentof relationships prior to measuring the relative elevation. Without suchcorrelations, the estimate of the relative elevation could be in errorby as much as eight to ten inches. The inventors have discovered thatnew membranes in such systems exhibit hysteresis. Such hysteresisappears to diminish as the membranes are repeatedly flexed. Therefore,the calibration may be performed after other startup procedures thatflex the membrane are completed.

[0022] Referring to FIG. 2 for the various referenced items, thepressure in at least one pump 300 or 310 is calibrated in process 401.In process 402, fluid communication is established between the at leastone pump 300 or 310 and the line 204. The static pressure in the pump incommunication with the line is measured in process 403, and the relativeelevation between the line and the fluid flow control device isestimated in process 404. This is accomplished by using the staticpressure measured in the pump and by using a known relationship betweena height differential and pressure differential (e.g., 0.3 m (1 foot)per 25 mm Hg.) Finally, the pump pressure may be adjusted to accommodatethe height differential in process 405.

[0023]FIG. 5(a) shows a block diagram highlighting the start up andcalibration procedures according to a preferred embodiment of theinvention that employs the fluid flow control system of FIG. 2. Theliquid volumes 302 and 312 are emptied in process 501 since either oneor both volumes may be partially full from previous procedures. Afterboth liquid volumes 302 and 312 are emptied, both outlet valves 306 and316 are closed in process 502. Both liquid volumes 302 and 312 aregravity filled with fluid in process 503. In this embodiment, the fluidis obtained through an inlet line from heated solution bag 202 as shownin FIG. 1. Since the solution bag 202 sits on top of the fluid flowcontrol device 200, the static head resulting from the difference inelevation between the solution bag 202 and the pumps 300 and 310 isknown and small. Liquid volumes 302 and 312 are filled via gravity assolution bag 202 is located at a higher elevation than are liquidvolumes 302 and 312.

[0024] The pressures in the control volumes 301 and 311 are measured inprocess 504 using the pressure transducers 305 and 315. A transducer maybe any instrument for converting a pressure to a signal, preferably anelectrical signal. The pressures measured in process 504 are to becorrelated to a zero static head (or defined to be zero). Both pumpinlet valves 307 and 317 are closed and the occluder bar 205 isactivated in process 505. The order of closure and activation is notsignificant. The effect of process 505 is to isolate the fluid in theliquid volumes 302 and 312 and outlet line 204 upstream of the occluderbar 205. In process 506 both outlet valves 306 and 316 are opened.Opening both outlet valves 306 and 316 enables fluid to be pumpedbetween the two liquid volumes 302 and 312 while keeping the total fluidvolume within the liquid volumes 302 and 312 constant.

[0025] The first control volume 301 is pressurized in process 507 to apre-selected positive pressure. The selection of the pre-selectedpositive pressure is determined by factors such as the expected range ofrelative elevations and the dynamic range of the pressure transducers305 and 315. The pressure in the first control volume 301 simulates aknown static head. The pressure in the second control volume 311 ismeasured in process 508 by the pressure transducer 315.

[0026] By assuming that the two membranes are identical in their effecton pressure transmission, a relationship is derivable from the two setsof pressure measurements, and calibration constants may be calculated atthis point. However, in further preferred embodiments of the invention,processes 507 and 508 are repeated. This time, the first control volume301 is pressurized using a pre-selected negative pressure in process 509and the pressure in the second control volume is measured in process510. In process 511 of this embodiment, a relationship is derived fromthe three sets of pressure measurements, and calibration constants arecalculated.

[0027]FIG. 5(b) is a block diagram illustrating the processes followingthe start up and calibration procedures of FIG. 5(a). In process 512 theliquid volumes 302 and 312 are again gravity filled. The valved outletis adjusted in process 513 allowing fluid communication only between thesecond pump 310 and the outlet line 204. The adjustment is accomplishedby emptying the first control volume 301, closing the first outlet valve306, and deactivating the occluder bar 205. The result of these actionsplaces the second pump 310 in fluid communication with the outlet line204 while isolating the second pump 310 from the rest of the system. Inprocess 514, the pressure transducer 315 measures the pressure in thesecond control volume 311, and the relative elevation is estimated inprocess 515 based on the pressure in the second control volume and thecalibration constants generated during calibration.

[0028]FIG. 6 is a graphical representation of two piecewise linear fitsusing coordinates of relative elevation (on the ordinate) and thepressure measured in process 514, P(control volume) (on the abscissa).The head pressure may be determined from the relative elevation by theequation p=ρgh, where p is the pressure, ρ is the fluid density, g isthe acceleration due to gravity, and h is the relative elevation. Apoint, H (on the ordinate), is determined by erecting a perpendicularfrom the P(control volume) value on the abscissa to the linear fitderived from the six calibration pressure values. Subsequently, thepressure at the distal end 208 of the fluid line 204, P(distal end), dueto the elevation differential may be calculated in process 516. Finally,the pressure in the pumps 300 and 310 may be adjusted in process 517 toaccommodate the height differential.

[0029] A computer program product may be employed for implementing themethods of the present invention. The computer program product comprisesa computer usable medium having computer readable program code thereon.The computer readable program may include program code for reading andstoring pressure values within the liquid volume 302 or 312, programcode for computing and identifying correlations between stored pressurevalues, program code for calculating pressure values based on theidentified correlations, and program code for estimating the elevationdifferential based upon the calculated pressures. The computer programproduct may also include program code for calculating a desired fluidpump pressure based upon the elevation differential and program code foradjusting the pump pressure in accordance with the desired pumppressure.

[0030] The computer program product may be run on a data processingunit, which acts as a controller. Such a unit may be capable ofadjusting the flow rate of fluid being pumped to the distal end 208 byadjusting the pump pressure. For example, if the calculation determinedthat the distal end 208 of the fluid line 204 and the fluid controlsystem were at the same height, the pump pressure might be safelyincreased above 75 mm Hg resulting in faster fluid flow rate. Further,all method processes may be performed under processor control. A memorymay be provided to store upper limits on safe pressures at the distalend 208 of the line 204 based upon the elevation differential betweenthe distal end 208 and the system. A processor capable of receiving dataas to elevation differential could then calculate and control pressurelevels.

[0031] Although, in the system described herein above, the liquidvolumes used to determine the relative elevation are pumps containingmembranes, it will appreciated that separate pumps, control volumes, andliquid volumes may be provided and that the liquid volumes and controlvolumes may be located at a different point from the pumps along thefluid pathway to the distal end of the fluid line. In such anembodiment, the height difference between the liquid volumes and thepumps or control volumes should be constant, so that the heightdifference is known. It should also be appreciated that the liquidvolumes and the pressurizing means need not be at the same location, andthat the first and second liquid volumes may likewise be in separatelocations.

[0032] It will be further understood by one of ordinary skill in the artthat other modifications can be made without departing from the spiritand the scope of the invention, as set forth in the claims below.

What is claimed is:
 1. A method for regulating fluid pump pressures, themethod comprising: providing a fluid flow control device, the fluid flowcontrol device having at least one liquid volume in valved communicationwith a distal end of a fluid line; calibrating the pressure in theliquid volume; establishing communication between the liquid volume andthe distal end; measuring a pressure associated with the liquid volume;and adjusting the fluid pump pressure in accordance with the measuredpressure.
 2. A method according to claim 1, wherein calibrating theliquid volume comprises: isolating the liquid volume from the distalend; measuring a first calibration pressure of a control volume inpressure communication with the liquid volume; pressurizing the liquidvolume; and measuring a second calibration pressure of the controlvolume.
 3. A method according to claim 1, wherein calibrating the liquidvolume comprises: isolating the liquid volume from the distal end;measuring the pressure of a control volume in pressure communicationwith the liquid volume; positively pressurizing the liquid volume;measuring the pressure of the control volume while the liquid volume ispositively pressurized; negatively pressurizing the liquid volume; andmeasuring the pressure of the control volume while the liquid volume isnegatively pressurized.
 4. A method according to claim 1, wherein theliquid volume includes a flexible membrane, which separates the liquidvolume from a control volume where pressure measurements are made.
 5. Amethod according to claim 1, wherein the fluid flow control deviceincludes a pump.
 6. A method for regulating fluid pump pressures, themethod comprising: providing a fluid flow control device, the fluid flowcontrol device having a first liquid volume and a second liquid volume,the first liquid volume in valved communication with the second liquidvolume; providing a fluid line, the fluid line having a distal end invalved communication with the first liquid volume and the second liquidvolume; calibrating the pressures in the liquid volumes; establishingcommunication between the first liquid volume and the distal end;measuring a pressure associated with the first liquid volume; adjustingthe fluid pump pressure in accordance with the measured pressure.
 7. Amethod according to claim 6, wherein calibrating the liquid volumescomprises: isolating the liquid volumes from the distal end; measuringfirst calibration pressures associated with the first liquid volume andthe second liquid volume; pressurizing the first liquid volume;measuring a second calibration pressure associated with the secondliquid volume; and adjusting the fluid pump pressure in accordance withthe measured pressures.
 8. A method according to claim 6, whereincalibrating the liquid volumes comprises: isolating the liquid volumesfrom the distal end; measuring first calibration pressures associatedwith the first liquid volume and the second liquid volume; positivelypressurizing the first liquid volume; measuring a second calibrationpressure associated with the second liquid volume while the first liquidvolume is positively pressurized; negatively pressurizing the firstliquid volume; measuring a third calibration pressure associated withthe second liquid volume while the first liquid volume is negativelypressurized; and adjusting the fluid pump pressure in accordance withthe measured pressures.
 9. A method according to claim 6, wherein eachof the first and second liquid volumes includes a flexible membrane thatseparates the liquid volume from a control volume, and wherein thepressure measurements are taken from the control volume.
 10. A methodaccording to claim 6, wherein the first and second liquid volumes aredisposed within a cassette.
 11. A method according to claim 10, whereinthe fluid flow control device includes a pump.
 12. A method forestimating the relative elevation between a first location and a secondlocation, the method comprising: placing a fluid flow control device atthe first location, the fluid flow control device having a membrane pumpin valved communication with the second location, the membrane pumphaving a pressure transducer; isolating the membrane pump from thesecond location; calibrating the pressure transducer; establishingcommunication between the membrane pump and the second location;measuring the pressure in the membrane pump with the pressuretransducer; and estimating the relative elevation based on the pressure.13. A method according to claim 12, wherein calibrating the pressuretransducer comprises: filling the membrane pump with fluid in pressureequilibrium with the pressure at the first location; measuring a firstcalibration pressure in the membrane pump; filling the membrane pumpwith fluid in pressure equilibrium with a known calibration pressure;measuring a second calibration pressure in the membrane pump; andestimating the relative elevation by constructing a linear fit betweenpressure and relative elevation based on the known calibration pressure,the first calibration pressure, and the second calibration pressure. 14.A method according to claim 13, wherein the relative elevation isdetermined by a processor.
 15. A computer program product for use on aprocessor for determining an elevation differential between a firstlocation and a second location, wherein a fluid flow control device islocated at the first location, the fluid flow control device includingat least one liquid volume in valved communication with a secondlocation via a distal end of a fluid line, the computer program productcomprising a computer usable medium having computer readable programcode thereon, the computer readable program code including: program codefor reading and storing pressure values within the pumps; program codefor computing and identifying correlations between stored pressurevalues; program code for calculating pressure values based on theidentified correlations; and program code for estimating the elevationdifferential based upon the calculated pressures.
 16. The computerprogram product according to claim 15, further comprising: program codefor calculating a desired pumping pressure based upon the elevationdifferential; and program code for adjusting the pumping pressuredwithing the fluid flow control device in accordance with the desiredpumping pressure.
 17. A computer program product for use on a processorfor regulating fluid pump pressures, wherein a fluid flow control deviceis located at the first location, the fluid flow control deviceincluding at least one liquid volume in valved communication with asecond location via a distal end of a fluid line, the computer programproduct comprising a computer usable medium having computer readableprogram code thereon, the computer readable program code including:program code for reading and storing pressure values within the pumps;program code for computing and identifying correlations between storedpressure values; program code for calculating pressure values based onthe identified correlations; and program code for calculating a desiredpumping pressure based on the identified correlations.
 18. A fluid flowcontrol device for detecting an elevation differential between a firstlocation and a second location, wherein the device is located at thefirst location, the fluid flow control device comprising: at least oneliquid volume; a fluid line in valved pressure communication with theliquid volume, the fluid line having a distal end located at the secondlocation; at least one transducer for measuring pressure associated withthe liquid volume; and a processor for: reading and storing pressurevalues; computing and identifying a correlation between the pressurevalues; calculating pressure values based on the identifiedcorrelations; and estimating the elevation differential based upon thecalculated pressures.
 19. A device according to claim 18, wherein thecorrelation computed and identified is a linear fit.
 20. A deviceaccording to claim 18, wherein the liquid volume includes a flexiblemembrane that separates the liquid volume from a control volume, andwherein the pressure measurements are taken from the control volume. 21.A device according to claim 18, wherein the liquid volume is disposedwithin a cassette.
 22. A device according to claim 18, furthercomprising at least one pump.
 23. A device according to claim 18,further comprising at least one piston.
 24. A system for regulatingfluid pump pressures, the system comprising: a fluid flow control devicehaving at least one liquid volume in valved communication with a distalend of a fluid line, the fluid flow control device including pressuremeans for pressurizing the liquid volume; a valve arrangement forcontrolling fluid communication between the at least one liquid volumeand the distal end; a transducer for measuring a pressure associatedwith the at least one liquid volume; and a controller for controllingthe valve arrangement, the means for pressurizing the liquid volume, andthe fluid pump pressure, and wherein the controller adjusts the fluidpump pressure in accordance with the pressure measured by thetransducer.
 25. A system for regulating fluid pump pressures, the systemcomprising: a fluid flow control device having first and second liquidvolumes in valved communication with each other and with a distal end ofa fluid line, the fluid flow control device including means forpressurizing at least one liquid volume; a valve arrangement forcontrolling fluid communication between the liquid volumes and betweenthe liquid volumes and the distal end; a transducer for measuring apressure associated with at least one liquid volume; and a controllerfor controlling the valve arrangement, the means for pressurizing theliquid volumes, and the fluid pump pressure, and wherein the controlleradjusts the fluid pump pressure in accordance with the pressure measuredby the transducer.
 26. A system according to claim 25, wherein thecontroller causes: the valve arrangement to isolate the liquid volumesfrom the distal end; the transducer to measure the pressure associatedwith the first liquid volume, while it is isolated from the distal end;the means for pressurizing the second liquid volume; and measuring thepressure associated with the first liquid volume after the pressure inthe second liquid volume has been pressurized.
 27. A system according toclaim 25, wherein the liquid volumes include a flexible membrane.
 28. Asystem according to claim 27, wherein at least one liquid volume isdisposed within a cassette.